1. Field of Invention
The present invention relates to a method of forming holes. More particularly, the present invention relates to a method of forming holes in a layer through a cross-shape image exposure.
2. Description of Related Art
In recent years, many semiconductor manufacturers design circuit devices with the specific aim of miniaturization. One very important process in semiconductor production is photolithography. Size of critical dimension (CD) in many semiconductor device structures such as various thin film patterns depends on photolithographic processes. In other words, photolithographic process has tremendous impact the future development of semiconductor devices. The transfer of pattern through photomask is especially important because inaccuracy in pattern transfer is likely to affect tolerance of critical dimensions on a chip and lower exposure resolution.
When a pattern on a photomask is transferred to a chip, proximity effect is a factor that has the most effect on the precision of critical dimension on the surface of a chip. Proximity effect occurs when a beam of light passes through a patterned photomask to form an image on the chip. The beam of light passing through the photomask is subjected to diffraction so that the beam is optically distorted. In addition, the beam of light may pass through a photoresist layer on the surface of the chip and then back-reflect from the surface of the substrate leading to optical interference and double exposure. Consequently, the intensity of exposure picked up by the photoresist layer is likely to change. This phenomenon occurs most frequently when the line width is very close to the wavelength used in the light source.
At present, to form a circular or elliptical hole in a chip, a two-in-one exposure method is often used to overcome the aforementioned exposure problem. FIGS. 1A to 1C are schematic perspective views showing the steps for producing conventional holes on a chip.
As shown in FIG. 1A, a dielectric layer 102 is formed on a substrate 100. A photoresist layer 104 is formed over the dielectric layer 102. Thereafter, a first exposure is carried out using a first photomask (not shown) to form an image on the photoresist layer 104. The first photomask has a rectangular shaped pattern thereon. After photo-exposure of the first photomask, a first pattern 106 comprising of two corner-laid regions is transferred to the photoresist layer 104.
As shown in FIG. 1B, a second exposure is carried out using a second photomask (not shown) to form an image on the photoresist layer 104. Similarly, the second photomask has a rectangular shaped pattern thereon. After photo-exposure of the second photomask, a second pattern 108 comprising of two corner-laid regions is transferred to the photoresist layer 104. The first pattern 106 and the second pattern 108 are mirror images of each other.
As shown in FIG. 1C, photolithographic and etching processes are conducted to remove the dielectric layer 102 underneath the first pattern 106 and the second pattern 108. Finally, the photoresist layer 104 is removed to form holes 110 in the dielectric layer 102a. 
However, the aforementioned method of forming holes may lead to a peeling of the photoresist layer 104 in a cylindrical region 112 (refer to FIG. 1B) between the first and the second pattern due to a leakage of light through the respective photomask in the first and the second photo-exposure. After etching, some defects may be produced in the dielectric layer 102a. To prevent such occurrences, a minimal distance between neighboring contact holes must be set. With the setting of a minimal distance between contact holes, a maximum pattern density is laid down preventing any further miniaturization of semiconductor devices.